Modernize file storage engine selection

Summary:
Fixes T5843. File storage engines use a very old "selector" mechanism which makes them difficult to extend.

This mechanism predates widespread use of `PhutilSymbolLoader` to discover available implementations at runtime. Runtime discovery has generally proven more flexible and easier to use than explicit selection (although it sometimes needs more UI to support it in cases where order or enabled/disabled flags can not be directly determined).

Use a modern runtime discovery mechanism instead of an explicit selector. This might break any installs which subclassed the `Selector`, but I believe almost no such installs exist, and they'll receive a meaningful exception upon upgrading (any custom engines will no longer implement all of the required methods).

Looking forward, this modernizes infrastructure to prepare for new "virtual" chunked-storage engines, with the eventual goal of supporting very large file uploads and data import into the Phacility cluster.

This uses D12051 to add UI to make it easier to understand the state of storage engines.

Test Plan:
Used new UI panel to assess storage engines:

{F336270}

  - Uploaded a small file, saw it go to MySQL engine.
  - Uploaded a larger file, saw it go to S3 engine.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5843

Differential Revision: https://secure.phabricator.com/D12053

src/docs/user/configuration/configuring_file_storage.diviner

@@ -3,32 +3,35 @@
 
 Setup how Phabricator will store files.
 
-= Overview =
+Overview
+========
 
 Phabricator allows users to upload files, and several applications use file
 storage (for instance, Maniphest allows you to attach files to tasks). You can
-configure several different storage systems:
+configure several different storage systems.
 
-  - you can store data in MySQL: this is the easiest to set up, but doesn't
-    scale well;
-  - you can store data on local disk: this is also easy to set up but won't
-    scale to multiple web frontends without NFS;
-  - or you can build a custom storage engine.
+| System | Setup | Cost | Notes |
+|========|=======|======|=======|
+| MySQL  | Automatic | Free | May not scale well. |
+| Local Disk | Easy | Free | Does not scale well. |
+| Amazon S3 | Easy | Cheap | Scales well. |
+| Custom | Hard | Varies | Implement a custom storage engine. |
 
 By default, Phabricator is configured to store files up to 1MB in MySQL, and
-reject files larger than 1MB. It is recommended you set up local disk storage
-for files larger than 1MB. This should be sufficient for most installs. If you
-have a larger install or more unique requirements, you may want to customize
-this further.
+reject files larger than 1MB. To store larger files, you can either:
 
-For technical documentation (including instructions on building custom storage
-engines) see @{article:File Storage Technical Documentation}.
+  - configure local disk storage; or
+  - configure Amazon S3 storage; or
+  - raise the limits on MySQL.
+
+See the rest of this document for some additional discussion of engines.
 
 You don't have to fully configure this immediately, the defaults are okay until
 you need to upload larger files and it's relatively easy to port files between
 storage engines later.
 
-= Storage Engines =
+Storage Engines
+===============
 
 Builtin storage engines and information on how to configure them.
 
@@ -41,23 +44,24 @@ Builtin storage engines and information on how to configure them.
 MySQL storage is configured by default, for files up to (just under) 1MB. You
 can configure it with these keys:
 
-  - ##storage.mysql-engine.max-size##: Change the filesize limit. Set to 0
+  - `storage.mysql-engine.max-size`: Change the filesize limit. Set to 0
     to disable.
 
-For most installs, it is recommended you configure local disk storage below,
-and then either leave this as is or disable it, depending on how upset you feel
-about putting files in a database.
+For most installs, it is reasonable to leave this engine as-is and let small
+files (like thumbnails and profile images) be stored in MySQL, which is usually
+the lowest-latency filestore.
+
+To support larger files, configure another engine or increase this limit.
 
 == Local Disk ==
 
   - **Pros**: Very simple. Almost no setup required.
   - **Cons**: Doesn't scale to multiple web frontends without NFS.
 
-For most installs, it is **strongly recommended** that you configure local disk
-storage. To do this, set the configuration key:
+To upload larger files:
 
-  - ##storage.local-disk.path##: Set to some writable directory on local disk.
-    Make that directory. You're done.
+  - `storage.local-disk.path`: Set to some writable directory on local disk.
+    Make that directory.
 
 == Amazon S3 ==
 
@@ -70,11 +74,6 @@ To enable file storage in S3, set these key:
   - ##amazon-s3.secret-key## Your AWS secret key.
   - ##storage.s3.bucket## S3 bucket name where files should be stored.
 
-== Custom Engine ==
-
-For details about writing a custom storage engine, see @{article:File Storage
-Technical Documentation}.
-
 = Testing Storage Engines =
 
 You can test that things are correctly configured by going to the Files